Many gas turbine engine fuel supply systems include a fuel source, such as a fuel tank, and one or more pumps that draw fuel from the tank and deliver pressurized fuel to the fuel manifolds in the engine combustor via a main supply line. The main supply line may include one or more valves in flow series between the pumps and the fuel manifolds. These valves generally include, for example, a main metering valve and a pressurizing-and-shutoff valve downstream of the main metering valve. In addition to the main supply line, many fuel supply systems also include a bypass flow line connected upstream of the metering valve that bypasses a portion of the fuel flowing in the main supply line back to the inlet of the one or more pumps, via a bypass valve. The position of the bypass valve is typically controlled by a head regulation scheme to maintain a substantially fixed differential pressure across the main metering valve.
The above-described fuel supply system is generally safe, reliable, and robust. Nonetheless, it can suffer certain drawbacks. For example, the metering valve and bypass valve can add to overall system weight and complexity. Moreover, control of the metering valve and bypass valve can result in increased system complexity and cost. Thus, in recent years there has been a desire to implement fully electric fuel controls by, for example, controlling the speed of a fuel metering pump. Yet, such efforts have been impeded because of the desire to maintain at least certain functions and features of present, non-fully electric fuel supply and control systems.
For example, most non-fully electric fuel supply and control systems are designed for certain postulated events that may result in certain postulated off-nominal operational modes, which in turn may result in certain postulated effects. One such postulated event is a loss of power. To accommodate this postulated event, the fuel supply and control system is typically designed such that, in the highly unlikely occurrence of a loss of power, the fuel metering valve “fails fixed.” That is, the fuel metering valve will remain in the position it was in when the postulated loss of power event occurs. As a result, fuel flow to the engine will remain at the flow rate that was commanded when the postulated loss of power event occurred.
Hence, there is a need for an electric fuel supply and control system that will implement a “fail fixed” feature in the unlikely event of a loss of power or other postulated event(s) that could prevent or inhibit fuel flow. Moreover, although a “fail fixed” configuration is generally safe and reliable, it is additionally desirable that the pilot be provided with a means to manually manipulate fuel flow to the engine during one of these postulated events. The present invention addresses at least these needs.